U.S. patent application number 11/650217 was filed with the patent office on 2007-06-07 for methods of preparing polysilynes.
Invention is credited to Patricia A. Bianconi, Scott Joray, Michael W. Pitcher.
Application Number | 20070129519 11/650217 |
Document ID | / |
Family ID | 35614031 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070129519 |
Kind Code |
A1 |
Bianconi; Patricia A. ; et
al. |
June 7, 2007 |
Methods of preparing polysilynes
Abstract
The invention involves new syntheses for poly(methyl- and
ethyl-silyne). The invention also includes silicon carbide (SiC)
ceramics that can be produced from poly(methylsilyne), as well as
other ceramics, which can be produced from these precursors by
modified processing conditions.
Inventors: |
Bianconi; Patricia A.;
(Sunderland, MA) ; Pitcher; Michael W.; (Davis,
CA) ; Joray; Scott; (Superior, CO) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
35614031 |
Appl. No.: |
11/650217 |
Filed: |
January 5, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11285372 |
Nov 22, 2005 |
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11650217 |
Jan 5, 2007 |
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10394827 |
Mar 21, 2003 |
6989428 |
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11285372 |
Nov 22, 2005 |
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60366851 |
Mar 22, 2002 |
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Current U.S.
Class: |
528/14 |
Current CPC
Class: |
C04B 41/009 20130101;
C04B 35/571 20130101; C04B 41/87 20130101; C09D 183/16 20130101;
C04B 41/5059 20130101; C04B 41/89 20130101; C08G 77/60 20130101;
C04B 41/52 20130101; C04B 41/52 20130101; C04B 41/5031 20130101;
C04B 41/52 20130101; C04B 41/4535 20130101; C04B 41/4554 20130101;
C04B 41/5059 20130101; C04B 41/5059 20130101; C04B 41/4535
20130101; C04B 41/4554 20130101; C04B 41/009 20130101; C04B 35/10
20130101; C04B 41/009 20130101; C04B 35/5618 20130101 |
Class at
Publication: |
528/014 |
International
Class: |
C08G 77/06 20060101
C08G077/06; C08G 77/04 20060101 C08G077/04 |
Claims
1. A method of making a poly(methylsilyne), the method comprising:
a) contacting a halogenated methylsilane with a metallic reagent to
produce a reaction mixture; b) homogenizing the reaction mixture to
produce a homogenized reaction mixture; c) adding to the
homogenized reaction mixture a solvent to aid in completing the
reaction; d) refluxing the homogenized reaction mixture for at
least about 6 hours to produce a first refluxed reaction mixture;
e) contacting the first refluxed reaction mixture with an
alkylating agent to produce an end-capped reaction mixture; f)
refluxing the end-capped reaction mixture to produce a second
refluxed reaction mixture; and h) quenching the second refluxed
reaction mixture with an aqueous solvent that lacks any alcohol to
produce non-pyrophoric poly(methylsilyne).
2. The method of claim 1, wherein the aqueous solvent is water.
3. The method of claim 1, wherein the solvent is
tetrahydrofuran.
4. The method of claim 1, wherein the halogenated methylsilane is
methyltrichlorosilane.
5. The method of claim 1, wherein the metallic reagent is a sodium
potassium alloy.
6. The method of claim 1, wherein the alkylating agent is
methylithium.
7. The method of claim 1, wherein the halogenated methyl silane is
mixed with a non-polar solvent.
8. The method of claim 7, wherein the non-polar solvent is
pentane.
9. The method of claim 1, wherein ultrasound is used to perform the
homogenization.
10. A method of making a poly(ethylsilyne), the method comprising:
a) contacting a halogenated ethylsilane with a metallic reagent to
produce a reaction mixture; b) homogenizing the reaction mixture to
produce a homogenized reaction mixture; c) slowly adding to the
homogenized reaction mixture a solvent, wherein at least 1.0 ml of
the solvent is added drop-wise, to aid in completing the reaction;
d) adding to the homogenized reaction mixture an alkylating agent
to produce an end-capped reaction mixture; and e) quenching the
end-capped reaction mixture with an aqueous solvent that lacks any
alcohol to produce non-pyrophoric poly(ethylsilyne).
11. The method of claim 10, wherein the aqueous solvent is
water.
12. The method of claim 10, wherein the solvent is
tetrahydrofuran.
13. The method of claim 1, wherein the halogenated ethylsilane is
ethyltrichlorosilane.
14. The method of claim 1, wherein the metallic reagent is a sodium
potassium alloy.
15. The method of claim 1, wherein the alkylating agent is
methylithium.
16. The method of claim 1, wherein the halogenated ethylsilane is
mixed with a non-polar solvent.
17. The method of claim 16, wherein the non-polar solvent is
pentane.
18. The method of claim 1, wherein ultrasound is used to perform
the homogenization.
19. The method of claim 10, further comprising refluxing the
homogenized reaction mixture.
20. The method of claim 10, further comprising refluxing the
end-capped reaction mixture.
21. A method of making a ceramic, the method comprising forming
poly(methylsilyne) by the method of claim 1; and heating the
poly(methylsilyne) to a temperature of at least 200.degree. C. to
form the ceramic.
22. The method of claim 21, wherein the poly(methylsilyne) is
heated to at least 1000.degree. C.
23. The method of claim 21, wherein the ceramic is within 5% of
stoichiometric.
24. The method of claim 21, wherein the poly(methylsilyne) is
heated by exposure to a plasma.
25. The method of claim 21, wherein the poly(methylsilyne) is
heated by exposure to a laser.
26. The method of claim 21, wherein the ceramic is silicon
carbide.
27. The method of claim 21, wherein the ceramic has a mean square
roughness of less than 200 .ANG., scanned over 5 microns.
28. A method of making a ceramic, the method comprising forming
poly(ethylsilyne) by the method of claim 1; and heating the
poly(ethylsilyne) to a temperature of at least 200.degree. C. to
form the ceramic.
29. The method of claim 28, wherein the poly(ethylsilyne) is heated
to at least 1000.degree. C.
30. A method of forming a film of poly(methylsilyne), the method
comprising forming poly(methylsilyne) by the method of claim 1;
solubilizing the poly(methylsilyne) in a solvent; and coating the
solubilized poly(methylsilyne) onto a substrate to form a film.
31. The method of claim 30, wherein the solvent is
tetrahydrofuran.
32. A method of forming a film of poly(ethylsilyne), the method
comprising forming poly(ethylsilyne) by the method of claim 10;
solubilizing the poly(ethylsilyne) in a solvent; and coating the
solubilized poly(ethylsilyne) onto a substrate to form a film.
33. The method of claim 32, wherein the solvent is tetrahydrofuran.
Description
RELATED APPLICATION
[0001] This application is a continuation (and claims the benefit
of priority under 35 U.S.C. .sctn.120) of U.S. application Ser. No.
11/285,372, filed Nov. 22, 2005, which claims the benefit of U.S.
application Ser. No. 10/394,827, filed Mar. 21, 2003, which claims
the benefit of U.S. Provisional Application Ser. No. 60/366,851,
filed Mar. 22, 2002, the contents of all of which are incorporated
herein by reference in their entirety.
TECHNICAL FIELD
[0002] This invention relates to new methods of preparing
poly(methylsilynes) and poly(ethylsilynes).
BACKGROUND
[0003] Silicon carbide (SiC) has been used in applications
requiring a hard, lightweight, temperature-and-wear-resistant
material. SiC has good fracture strength, hardness, low theoretical
density (p=3.21 g/cc) and thus relatively high strength/weight
ratio. It is an attractive material for numerous applications.
Conventionally produced SiC materials are manufactured using SiC
powder processing and sintering. In this process, forming shaped
products can be difficult and typically requires temperatures in
excess of 2100.degree. C. A number of chemical approaches based on
polymer precursors have been developed for the synthesis of SiC
(see for example, Laine et al., Chem. Mater. (1993), 5, 260;
Richter et al., Applied Organometallic Chemistry (1997), 11, 71;
Seyferth, Adv. Chem. Ser. (1995), 245, 131; and Birot et al. Chem.
Rev. (1995), 95, 1443). Polymer precursors can offer some
advantages over the conventional solid-state processing of SiC.
However, some polymers lack the needed degree of processability or
require difficult syntheses. The low char yields of most precursors
lead to excessive shrinkage and cracking in the ceramic products
and deterioration of mechanical properties. The ceramics are often
rich in either Si or C, which again may lead to a degradation of
the desired properties.
[0004] Polymethylsilane (PMS) syntheses give high-yield, near
stoichiometric SiC ceramics upon pyrolysis. The syntheses involve
producing pyrophoric polymers with the formula
(CH.sub.3Si).sub.x(CH.sub.3SiH).sub.y, which must be further
crosslinked by some mechanism (for example, borate
(B[OSi(CH.sub.3).sub.3].sub.3) thermolysis) to give ceramics in
high yield. Chain-terminating agents (such as (CH.sub.3).sub.3SiCl)
have been added to such systems, allowing ceramic yields of up to
64%. Polyvinylsilane has been added to PMS as a further pyrophoric
ceramic precursor. Stabilization with 2,6 di-t-butyl-4-methylphenol
(BHT) is generally required for all pyrophoric PMS syntheses.
Syntheses of this type tend to be multistep and fairly complex.
[0005] Polysilynes were synthesized by Bianconi and Weidman in 1988
(Bianconi et al., J. Am. Chem. Soc. (1988), 110, 2342). The
synthesis generally involves the reduction of alkyl- or arylsilicon
trihalides with liquid NaK. High intensity ultrasound is used to
ensure rapid and a more homogeneous reaction environment. These
silicon-silicon bonded network polymers adopt a unique structure,
in which each silicon bears one pendant group and is joined by
three single bonds to three other silicon atoms, forming a
continuous random network backbone. These silicon network polymers
have a distinctive yellow color, very broad NMR resonances, and a
broad and intense UV absorption band edge tailing into the visible.
Recently Huang and Vermeulen have synthesized these network
polymers electrochemically (Chem. Commun. (1998), 247). However, it
has been reported that Wurtz coupling of methyltrichlorosilane
yields a white intractable solid, unsuitable for processing into
SiC (see Brough et al., J. Am. Chem. Soc. (1981), 103, 3049; West
et al., J. Am. Chem. Soc. (1972), 94, 6110; Matyjaszewski et al.,
Polymer Bulletin (1989), 22, 253; Bianconi et al., Macromolecules
(1989), 22, 1697; and Vermeulen et al., Polymer (2000), 41(2),
441.
SUMMARY
[0006] The invention is based, in part, on the discovery of new
methods of synthesis for poly(methylsilyne), (CH.sub.3Si).sub.n,
and poly(ethylsilyne), (CH.sub.2CH.sub.3Si).sub.n, and the use of
these new methods to make new non-pyrophoric poly(methyl- or
ethyl-silyne) silicon carbide precursors, as well as films and
ceramics made from these precursors.
[0007] In general, the invention features a novel synthesis of
poly(methyl- or ethyl-silyne) by a modified Wurtz-type coupling
mechanism. The reaction is straightforward and yields a
non-pyrophoric polymer, which can be used as an SiC preceramic
polymer. The polymer is soluble in tetrahydrofuran and other common
organic solvents, which enables the formation of films and fibers.
The ceramic resulting from the pyrolysis of poly(methyl- or
ethyl-silyne) is produced in high yields, and is a perfectly
stoichiometric SiC. The SiC ceramic film produced from the PMSy
precursor is smooth, continuous, and essentially defect free
compared to films produced by other known methods.
[0008] In general, the invention features methods of making
poly(methylsilyne) by contacting a halogenated methylsilane with a
metallic reagent to produce a reaction mixture; homogenizing the
reaction mixture to produce a homogenized reaction mixture; adding
to the homogenized reaction mixture a solvent to aid in completing
the reaction; refluxing the homogenized reaction mixture for at
least about 6 hours to produce a first refluxed reaction mixture;
contacting the first refluxed reaction mixture with an alkylating
agent to produce an end-capped reaction mixture; refluxing the
end-capped reaction mixture to produce a second refluxed reaction
mixture; and quenching the second refluxed reaction mixture with an
aqueous solvent that lacks any alcohol to produce non-pyrophoric
poly(methylsilyne).
[0009] In these methods, the aqueous solvent can be water, the
solvent can be tetrahydrofuran (THF), the halogenated methylsilane
can be methyltrichlorosilane, the metallic reagent can be a sodium
potassium alloy such as NaK, the alkylating agent can be
methylithium, the halogenated methyl silane can mixed with a
non-polar solvent (e.g., pentane), and ultrasound can be used to
perform the homogenization.
[0010] In similar methods, poly(ethylsilyne) can be made by
contacting a halogenated ethylsilane with a metallic reagent to
produce a reaction mixture; homogenizing the reaction mixture to
produce a homogenized reaction mixture; slowly adding to the
homogenized reaction mixture a solvent, wherein at least 1.0 ml of
the solvent is added drop-wise, to aid in completing the reaction;
adding to the homogenized reaction mixture an alkylating agent to
produce an end-capped reaction mixture; and quenching the
end-capped reaction mixture with an aqueous solvent that lacks any
alcohol to produce non-pyrophoric poly(ethylsilyne). Thus,
refluxing steps are not required, but can be included as in the
methods of making poly(methylsilynes), and the solvent, such as THF
must be added slowly. The specific components can be as listed
above, except that the halogenated ethylsilane can be
ethyltrichlorosilane.
[0011] In another aspect, the invention features methods of making
ceramics, such as silicon carbides, by forming poly(methylsilyne),
or poly(ethylsilyne), according to the methods described herein;
and heating the poly(methyl- or ethyl-silyne) to a temperature of
at least 200.degree. C. (e.g., at least 500, 750, 1000, 1500, or
1600.degree. C.) to form the ceramic. The ceramic can be within 5%
of stoichiometric, or can be substantially stoichiometric silicon
carbide, and the poly(methyl- or ethyl-silyne) can be heated by
exposure to a plasma or a laser. The new ceramics can have a mean
square roughness of less than 200 .ANG., scanned over 5 microns or
larger scan regions, such as 2 mm.
[0012] In another embodiment, the invention also features methods
of forming films of poly(methyl- or ethyl-silyne) by forming
poly(methyl- or ethyl-silyne) as described herein; solubilizing the
poly(methyl- or ethyl-silyne) in a solvent; and coating the
solubilized poly(methylsilyne) onto a substrate to form a film. The
substrate can be aluminum, and the solvent can be
tetrahydrofuran.
[0013] As used herein "substantially stoichiometric silicon
carbide" is material in which the atomic ratio of silicon to carbon
is within 5% of 1:1.
[0014] A non-pyrophoric polymer is one that does not ignite or
produce a spark when exposed to air.
[0015] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, suitable methods and materials are described below. All
publications, patent applications, patents, and other references
mentioned herein are incorporated by reference in their entirety.
In case of conflict, the present specification, including
definitions, will control. In addition, the materials, methods, and
examples are illustrative only and not intended to be limiting.
[0016] Other features and advantages of the invention will be
apparent from the following detailed description, and from the
claims.
DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a ultraviolet/visible spectrum of
poly(methylsilyne).
[0018] FIG. 2 is a Fourier Transform Infrared spectrum of
poly(methylsilyne).
[0019] FIG. 3 is a Fourier Transform Infrared spectrum of
poly(methylsilyne) with an extended reflux period and after
addition of iodomethane.
[0020] FIG. 4 is a .sup.29Si-NMR spectrum of
poly(methylsilyne).
[0021] FIG. 5 is a graph of % ceramic yield of SiC vs. molecular
weight of poly(methylsilyne).
[0022] FIG. 6 is a photograph of a silicon carbide film produced
according to a particular embodiment of the invention.
[0023] FIGS. 7A and 7B are photographs of a prior art silicon
carbide film produced from a polymethysilane precursor (from
Czubarow et al., Macromolecules, 31:229, 1998).
[0024] FIG. 8 is a photograph of a silicon carbide film produced
from poly(n-hexyl)silyne.
DETAILED DESCRIPTION
[0025] The invention provides a new and simple modified Wurtz
coupling reaction to prepare poly(methyl- or ethyl-silyne), which
polymers have previously been impossible to produce using standard
Wurtz coupling reactions. The reaction is straightforward and
yields a non-pyrophoric polymer, which can be used as an SiC
preceramic polymer. The polymer is soluble in tetrahydrofuran and
other common organic solvents, which enables the formation of films
and fibers. The ceramic resulting from the pyrolysis of
poly(methyl- or ethyl-silyne) is produced in high yields, and is a
perfectly stoichiometric SiC. The SiC ceramic films produced from
the precursors are smooth, continuous, and essentially
defect-free.
[0026] Methods of Making Poly(Methyl- or Ethyl-Silyne)
[0027] Poly(methylsilynes) (PMSy) and poly(ethylsilynes) (PEtSy)
are prepared by a modified Wurtz coupling reaction. Suitable
starting materials include alkyltrihalosilanes or
alkytrialkoxysilanes, such as, for example, methyltrichlorosilane,
methyltrimethoxysilane, ethyltrichlorosilane, or
ethyltrimethoxysilane. The starting material can be reacted with a
reagent used in the known Wurtz reaction such as silver, zinc,
activated copper, pyrophoric lead, lithium, complexed nickel,
potassium, sodium, or cesium metals. Alloys of these metals can
also be used. Suitable reagents include NaK, NaHg, KHg, NaKHg, and
similar alloys, in any ratio. For example, NaK in an approximately
1:1 ratio is suitable for this reaction. A solvent can optionally
be used. Nonprotic solvents including a heteroatom such as
nitrogen, sulfur, or oxygen can be employed. Such solvents can
assist in the formation of emulsions. Such solvents include ethers,
amines, including and not limited to dimethylsulfoxide,
acetonitrile, dimethylformamide, acetone, hexamethylphosphoramide,
tetrahydrofuran, diethyl ether, and many other similar solvents.
Solvents, if used, should be dried thoroughly. In addition, when
preparing poly(ethylsilyne), the solvent must be added very slowly,
e.g., drop-wise, for at least the first two milliliters of the
solvent, to prevent a violent reaction and possible explosion.
[0028] The reaction is carried out under an inert atmosphere.
Sonication is carried out to produce a homogeneous mixture, for
example, for at least about three minutes, e.g., at least about 5,
10, or 15 minutes.
[0029] After sonication is complete, the reaction mixture is
refluxed under an inert atmosphere for a time suitable to drive the
reaction to completion, e.g., at least about 6, 8, 10, 12, 18, 20,
or 24 hours. If this reflux is not performed in the synthesis of
PMSy, a white, intractable solid is formed. On the other hand, such
a reflux step has been found to degrade other polysilynes.
[0030] After the first reflux step, the polymer is end-capped by
exposure to an appropriate amount of an alkylating agent. Such
alkylating agents include, for example, alkyllithiums such as
methyllithium, and Grignard reagents such as methyl magnesium
bromide. After end-capping, the mixture is refluxed a second time
under an inert atmosphere for a further period to complete the
reaction, for example, for at least 6, 8, 10, 12, 18, 20, or 24
hours. Again, if this reflux step is not done in the synthesis of
PMSy, a white intractable solid is formed.
[0031] Quenching the reaction after the second reflux step directly
with water produces the desired yellow PMSy polymer. Handling of
the material under an inert atmosphere is no longer required after
quenching. Dehalocoupling reactions of halosilanes are typically
quenched with methanol, to consume any unreacted alkali metal.
Previously published polysilyne syntheses, and other silicon
polymer syntheses, use sequential precipitation from alcohols,
including methanol, as a purification step. However, we have found
that contact of the polymer with any alcohol causes instantaneous,
irreversible polymer degradation to a white intractable solid, and
thus must be avoided in the new methods described herein.
[0032] The polymers produced according to these methods have a
molecular weight from about 1,000 to about 20,000, e.g., about
1,000, 2,500, 5,000, 7,000, 10,000, or 15,000.
[0033] Uses of Poly(Methyl- or Ethyl-Silyne)
[0034] The polymers can be applied to surfaces using standard
coating techniques to form silicon carbide (SiC) films. The
polymers can be applied in any desired thickness, by dissolution to
any suitable concentration in any suitable solvent described above.
The polymers can also be spun onto surfaces, or applied to objects
according to conventionally known methods. The polymers can also be
formed into fibers by using fiber-pulling machines known to those
of skill in the art.
[0035] SiC formation takes place by heating, or by plasma- or
laser-induced processes. For example, heating to at least about
200.degree. C. can lead to forms of SiC that are incompletely
crystallized. Higher temperatures, e.g., about 500, 750, 850, 1000,
1200, or about 1500.degree. C., or increased pressures, e.g., 2 or
3 atmospheres, can produce SiC of a higher degree of crystallinity.
Plasma- or laser-induced SiC formation can take place at room
temperature, since local heating will produce SiC. Low temperature
SiC formation can also be assisted by adding seed crystals of
SiC.
[0036] The performance of ceramic SiC and the applications for
which it can be used are very much dependent on ceramic
composition. Crystalline forms of SiC can be desirable for use in
electronics applications, for example, for their thermal conduction
properties. Incompletely crystallized forms of SiC can be desirable
if films of SiC are to be produced on substrates which are
sensitive to the conditions required for producing substantially
crystalline SiC. Films of virtually any thickness can be produced.
Fibers can be desirably produced in incompletely crystallized
forms. Amorphous forms of SiC can also be desirable for
applications not requiring such demanding physical properties.
[0037] The highly pure SiC produced by these processes can be used
in electronics applications for protective coatings, and for
thermal transfer applications. Hard drive coatings can be made
including SiC. This material can also be incorporated into commonly
used items such as boat hulls or tennis racquets. In addition,
poly(methylsilyne) can be used to create bulk objects such as
molded objects.
[0038] Formation of SiC by heat or plasma processing, or under
chemically reactive atmospheres (such as NH.sub.3, H.sub.2,
CH.sub.4 or SiH.sub.4), can be used to tailor the new ceramic
compositions to form Si.sub.xC.sub.yN.sub.z and can turn PMSy into
a ceramic precursor of unrivalled versatility. Processing under
methane, for example, can alter the Si:C ratio further toward
carbon or to create carbon rich Si.sub.xC.sub.y material. This
excess silicon or carbon is incorporated into the ceramic, and not
present as elemental silicon or carbon. Processing under hydrogen
gas allows scavenging of hydride or excess carbon to alter the Si:C
ratio toward silicon. Processing under silane can also alter the
Si:C ratio to increase the silicon content of the material.
Processing under ammonia may introduce Si--N into the silicon
carbide, which is generally conventionally possible by
sputtering.
[0039] The invention will be further described in the following
examples, which do not limit the scope of the invention described
in the claims.
EXAMPLES
[0040] The following examples illustrate particular advantages and
properties of the materials and methods described herein. All the
reactions were carried out under an argon atmosphere, by means of
standard Schlenk manipulations or inside a glove box. Anhydrous
pentane and tetrahydrofuran were purchased from Aldrich and were
dried over sodium metal and benzophenone and distilled prior to
their use. Methyltrichlorosilane (99%) was purchased from VWR and
used as received. Methyllithium (1.4M in diethyl ether) was
purchased from Aldrich and used as received. Liquid 1:1 mole ratio
NaK alloy was prepared in a glove box by adding solid potassium to
an equimolar amount of molten sodium.
[0041] .sup.1H NMR (200.1 MHz) spectra were recorded on a Bruker
AC200.RTM.. .sup.13C NMR (75.4 MHz) spectra were recorded on a
Bruker DPX300.RTM.. .sup.29Si NMR (99.4 MHz) spectra were recorded
on a Bruker AMX500.RTM., using a Bruker 5 mm broadband direct
probe. A distortion-less enhanced proton transfer-45 (DEPT45)
sequence was run with J=7 Hz. In all cases, d.sup.8-tetrahydrofuran
was used as the solvent at room temperature. FTIR transmission
spectra were obtained using a Midac.RTM. M12-SP3 spectrometer,
operating at 4 cm.sup.-1 resolution with neat film samples between
salt plates or with KBr pellets. Oxygen incorporation studies were
done using a Rayoner.RTM. RPR-100 photochemical reactor. UVN is
spectra were measured at room temperature, in 3.times.10.sup.-4 M
cyclohexane solution using a Shimadzu RUV-260.RTM. spectrometer.
The molecular weights of the polymers were determined versus
polystyrene standards on a Polymer Labs LC1120.RTM. HPLC pump,
fitted with an IBM LC9563 UV detector, using tetrahydrofuran as a
solvent.
[0042] As a control, poly(n-hexyl)silyne (PNHS) was synthesized
using the method of Bianconi et al., J. Am. Chem. Soc., 7, 2342
(1988). Pyrolysis studies of PMSy and poly(n-hexyl)silyne (PnHS)
were performed using a Thermolyne 12110.RTM. tube furnace; all
studies were done under a dynamic argon flow and a heating rate of
10.degree. C./minute. Ceramic yields are quoted as percentage
weight retention. Films of PMSy and PnHS were spun at 1000 rpm for
10 minutes on AlTiC substrates with an alumina basecoat, on a
Headway Research Inc. Photo Resist spinner model
1-EC101DT-435.RTM., from a 0.2 g/mL PMSy/THF solution. Film
thickness and roughness measurements were obtained using a Tencor
Instruments Alpha Step 500 Surface Profiler.RTM.. Scanning electron
micrographs (SEM) were taken on a JOEL JSM-35CF.RTM. scanning
microscope. Energy Dispersive X-ray spectroscopy (EDS) was carried
out using a JOEL 6320 FXV scanning microscope configured with a PGT
Imix Xe X-ray microanalysis system; a 100-second collection time
was used for X-ray spectral analysis. Spectra were then quantified
as weight percents for Si and C. The XRD pattern was recorded on a
Siemens D-500 diffractometer in transmission geometry with a Ni
filtered CuK.alpha. radiation.
Example 1
Preparation of PMSy
[0043] An oven-dried 400 mL beaker containing anhydrous pentane
(250 mL) and 7.474 g (50 mmol) of methyltrichlorosilane, was placed
in a nitrogen atmosphere drybox equipped with a high intensity (475
W, 20 KHz, 1/2 inch tip) ultrasonic immersion horn. The solvent and
methyltrichlorosilane were irradiated at full power by immersion of
the horn for 3 minutes. 4.42 g (143 mmol) of NaK alloy was added
slowly drop-wise over a period of 5 minutes. Sonication was
continued for a further 8 minutes after addition was complete. 200
ml of THF was then added to the reaction mixture, and sonication
continued for a further 8 minutes.
[0044] At this time the dark blue reaction mixture was transferred
to reflux apparatus and transferred to a Schlenk line. The mixture
was refluxed gently for 24 hours, under a dynamic flow of argon, in
which time the reaction mixture had turned brown in color. At this
time, 7.0 mL of methylithium (1.4 M in diethyl ether) was added to
end-cap the PMSy polymer. Thereafter, reflux was continued for a
further 24 hours, again under a dynamic argon flow. 100 mls of
water were added with vigorous stirring to quench the reaction
mixture. There was no longer a need for an inert atmosphere at this
point. On transferring to a separating funnel, separation of the
aqueous and organic layers occurred. A yellow organic layer was
isolated from the clear aqueous layer and the solvent removed under
vacuum. Yields of 50-70% were typically obtained.
[0045] Characterization of poly(methylsilyne) was done by UV/Vis,
FTIR, .sup.1H, .sup.13C, .sup.29Si NMR spectroscopies, GPC, and by
elemental analyses. As indicated below, all data is consistent with
the formation of PMSy. It should be noted that PMSy is not
pyrophoric, providing a considerable advantage over many other SiC
polymer precursor systems.
[0046] FTIR (neat, cm.sup.-1 (assignment)): 2973, 2862 (.nu. C--H,
SiCH.sub.3), 2070 (.nu. Si--H) 1460, 1245 (.delta. C--H,
SiCH.sub.3), 1069 (.nu. Si--O--Si), 911 (.gamma. SiH.sub.2), 836
(.rho. CH.sub.3), 774, 685 (.nu. Si--C). .sup.13C NMR (ppm
assignment): -3.0, very broad, (SiCH.sub.3). .sup.29Si NMR (ppm
assignment): -74.5 (SiCH.sub.3), -66.3 (HSiCH.sub.3), -37.3 and
-33.1 (Si(CH.sub.3)H.sub.2), -21.4 and -15.5 (CH.sub.3SiCH.sub.3
(linear fragments and Si(CH.sub.3).sub.2 end groups), +8.1
((CH.sub.3).sub.3Si). .sup.1H NMR (ppm assignment): 0.37, very
broad (SiCH.sub.3), 3.45, broad, (SiH, SiH.sub.2). Elemental
analyses: Found (C, 29.14%; H, 8.37%; Cl, <0.2%); Calculated for
(CH.sub.3Si).sub.n (C, 27.85%; H, 7.01%).
[0047] The UV/Vis spectrum of PMSy is shown in FIG. 1. It shows a
broad and intense absorption in the UV, which tails off into the
visible (at about 500 nm). This feature is characteristic of
polysilynes and is attributed to extension of Si--Si
.sigma.-"conjugation" into three dimensions, and differentiates
polysilynes from linear polysilanes which exhibit strong
.sigma.-.sigma.* transitions (.lamda..sub.max=300-350 nm).
[0048] The FTIR spectrum of PMSy is shown in FIG. 2. This spectrum
is consistent with that expected for PMSy. It is notable that the
.nu. Si--H and .gamma. SiH.sub.2 bands are much less intense than
in recently reported polymer precursors to SiC. This is manifested
in the fact that the polymer is not pyrophoric and can be readily
handled in air, for short time periods. The .nu. Si--H (2070
cm.sup.-1) and .gamma. SiH.sub.2 (911 cm.sup.-1) bands can be
almost entirely eliminated by the addition of a couple of mls of
iodomethane to the reaction mixture (after addition of the MeLi)
and reflux for a further 24 hours (as shown in FIG. 3). Freshly
prepared PMSy also shows relatively few Si--O--Si moieties, as
evidenced by the sharp peak at 1069 cm.sup.-1. Typically, the
presence of large numbers of Si--O--Si units gives a broad
absorption in this region. The absence of residual Si--Cl in PMSy
as evidenced by the absence of bands in the 498-525 cm.sup.-1
region is also notable. When handled in air for prolonged times and
in the direct presence of UV light, PMSy becomes insoluble due to
the incorporation of oxygen into the Si--Si backbone.
[0049] The .sup.1H NMR spectrum (not shown) also confirms that the
product is almost entirely PMSy. The resonance at +3.45 ppm (SiH,
SiH.sub.2) is very small compared to the broad resonance at +0.37
ppm (SiCH.sub.3). Resonances above +5 ppm are not observed; these
would be attributable to SiHCl or SiOH. As mentioned, the SiH,
SiH.sub.2 signals can be removed by addition of iodomethane. The
.sup.13C NMR spectrum (not shown) indicates only the presence of
SiCH.sub.3 as expected.
[0050] The .sup.29Si NMR is shown in FIG. 4. The broad resonance at
-74.5 ppm is expected, and is attributable to methyl groups on the
silicon backbone. This spectrum also shows the presence of other
silicon moieties.
[0051] The elemental analysis of PMSy was very close to the
expected composition. It shows that the polymer is slightly rich in
both carbon and hydrogen, which is to be expected from FTIR,
.sup.1H and .sup.29Si NMR. Cl, <0.2% is consistent with all
other data. Si analyses are notoriously difficult to obtain from
these silicon polymers.
[0052] Gel permeation chromatography (GPC) analysis of the polymer
reveals polydispersity in PMSy. Generally, we formed polymers of
M.sub.w.apprxeq.7000 with a wide polydispersity (.apprxeq.4). We
also formed polymers with molecular weights up to about 20,000, and
with polydispersity of about 7. We have also formed brown insoluble
powders, and these may be even higher molecular weight versions of
PMSy, which would account for the insolubility.
[0053] Pyrolysis studies confirm that that the major weight loss
process for PMSy occurs in the region 200-450.degree. C. The
ceramic yield is very much dependent on the molecular weight of the
polymer, as shown in the graph in FIG. 5, which shows the
percentage ceramic yield of SiC vs. molecular weight of
poly(methylsilyne). In approximately 50% of the pyrolysed samples a
black-colored ceramic is obtained. However, in the remaining half
of the samples the ceramic displays light brown to pale yellow
coloration. This coloring is indicative of extremely high purity
silicon carbide, as observed by Greenwood and Earnshaw, Chemistry
of the Elements, Pergammon Press, New York, (1989), 386.
Example 2
Preparation of PEtSy
[0054] An oven-dried 400 mL beaker containing anhydrous pentane
(250 mL) and 7.474 g (50 mmol) of ethyltrichlorosilane, was placed
in a nitrogen atmosphere drybox equipped with a high intensity (475
W, 20 KHz, 1/2 inch tip) ultrasonic immersion horn. The solvent and
ethyltrichlorosilane were irradiated at full power by immersion of
the horn for 3 minutes. 4.42 g (143 mmol) of NaK alloy was added
slowly drop-wise over a period of 5 minutes. Sonication was
continued for a further 8 minutes after addition was complete. 200
ml of THF was then added to the reaction mixture very slowly, e.g.,
drop-wise for the first 2 or 3 mL, and sonication continued for a
further 8 minutes.
[0055] At this time the dark blue reaction mixture was transferred
to reflux apparatus and transferred to a Schlenk line. The mixture
was (optionally) refluxed gently for about 12 hours, under a
dynamic flow of argon, during which time the reaction mixture had
turned brown in color. At this time, 7.0 mL of methylithium (1.4 M
in diethyl ether) was added to end-cap the PEtSy polymer.
Thereafter, reflux was continued for a further 24 hours, again
under a dynamic argon flow. 100 mls of water were added with
vigorous stirring to quench the reaction mixture. There was no
longer a need for an inert atmosphere at this point. On
transferring to a separating funnel, separation of the aqueous and
organic layers occurred. A yellow organic layer was isolated from
the clear aqueous layer and the solvent removed under vacuum.
Yields of 50-70% were typically obtained.
Example 3
Preparation of Silicon Carbide (SiC)
[0056] PMSy heated to pyrolysis temperatures of 1000.degree. C.,
under argon, produced SiC in high yield (up to 85%, by weight loss,
which is close to the theoretical yield expected for this polymer).
During heating, the temperature was slowly ramped up at a rate of
10.degree. C./minute, and held at 250.degree. C. for about 2 hours
before continuing to increase the temperature.
[0057] The ceramic yield is very much dependent on the molecular
weight of the polymer, which is a well-known attribute of these
types of polymers. Purity is confirmed by elemental analysis of the
ceramic: C, 27.30; Si, 61.20; .SIGMA.=88.50; Calcd for SiC: C,
29.95; Si, 70.05. This equates to SiC with the formula SiC.sub.1.04
or a ceramic with the composition 1.08% C+98.92 SiC. Energy
dispersive spectroscopy (EDS) analysis of the ceramic film formed
from PMSy on the alumina substrate reveals the high purity of the
ceramic product: Found C, 29.95%; Si, 70.05%; Calcd for SiC: C,
29.95; Si, 70.05, or a ceramic with the composition of 100% SiC
with Si and C present in a perfect 1:1 ratio. Such analytically
pure ceramic has not been obtained from use of other polymer
precursor systems.
Example 4
Preparation of PMSy Films
[0058] Samples of PMSy were spun onto alumina substrates to obtain
uniform and smooth films of PMSy (2 .mu.m thick, mean square
roughness (Rq)=200-300 .ANG., scanned over 2 mm). Heating these
films to 1000.degree. C. produced smooth ceramic films of uniform
thickness (1 .mu.m thick, Rq=170 .ANG., scanned over 2 mm), as
measured by. A photograph of this material is shown in FIG. 6. The
smoothness indicates a dense, homogeneous ceramic film (in the area
scanned), without pores, cracks, or other defects; such
high-quality ceramic is not reported from use of other polymer
precursor systems, absent further polishing. The ceramic films
produced were adherent to the substrates, resistant to removal by
plastic adhesive tape, and were completely uniform.
Example 5
Comparisons with Other Films
[0059] FIGS. 7A and 7B are published photographs of an SiC film
made from a polymer precursor (polymethylsilane) heated to
1000.degree. C., under argon. As shown, the film had significant
defects and was not continuous. The figures are from Czubarow et
al., Macromolecules, 31, 229, (1998).
[0060] The improved ceramic-producing behavior of PMSy over other
polysilynes, such as poly(n-hexyl)silyne, is shown in FIG. 8. A
film of poly(n-hexyl)silyne is shown after being spun onto an
alumina substrate and pyrolysed. Severe cracks and inhomogeneities
are seen in the resulting ceramic film. This is believed to result
from loss of most of the mass of the n-hexyl side chain during
pyrolysis, leading to high weight loss and low ceramic yield.
OTHER EMBODIMENTS
[0061] It is to be understood that while the invention has been
described in conjunction with the detailed description thereof, the
foregoing description is intended to illustrate and not limit the
scope of the invention, which is defined by the scope of the
appended claims. Other aspects, advantages, and modifications are
within the scope of the following claims.
* * * * *